1. Field of the Invention
The present invention relates to an X-ray radiographing apparatus provided with a power assist device causing a driver mechanism to generate force to assist that force which is added to the handle assembly in a direction, so that a heavy component (such as the spot film device) is moved in the direction by the sum of both forces.
2. Description of the Related Art
When radiographing part of a patient is to be positioned, in the X-ray radiographing apparatus, the spot film device having an image intensifier is moved in a direction parallel to the tabletop of the apparatus or in the direction of its nearing to (or departing from) the bed. The X-ray radiographing apparatus is provided with a power assist device in this case. Force or power is added to the handle assembly of the spot film device by the operator in order to move the spot film device in a direction. The force added to the handle assembly is detected by a pair of sensors, e.g., pressure sensors. When signal responsive to the force detected is supplied to the driver mechanism, the driver mechanism generates force to assist the force which have been added to the handle assembly. The spot film device is moved by the sum of both forces. Therefore, the operator can move the relatively heavy spot film device (which weighs 100-150 kg, for example) only by adding faint force to the handle assembly.
The bed is erected vertical to the floor, and is kept in its posture, as shown in FIG. 3. The handle assembly is also made vertical to the floor in this case. This causes the pressure sensor positioned under the handle assembly to be wrongly sensed by the weight of the handle assembly. As the result, the spot film device is left moving under no control. The pressure sensor located above the handle assembly cannot be made operative if force larger than a certain value is not added to it. This makes the starting force of one pressure sensor different from that of the other one and the operator must therefore operate the handle assembly, taking this difference into consideration.
Each of the pressure sensors has, therefore, an electric blind zone not to be sensed by force smaller than the certain value. Their wrong sensing can be thus prevented.
Because the pressure sensors have electric blind zones, however, the spot film device cannot be moved at the instant when the operator adds force to the handle assembly, and it cannot be stopped at the instant when the operator stops adding force to the handle assembly. In other words, the response speed of the spot film device is low and its stop accuracy is not good. As the result, the operator must operate the handle assembly, taking it into consideration that the response speed of the spot device is low and that its stop accuracy is not good. This is a serious burden imposed on the operator.
Potentiometers are sometimes used instead of the pressure sensors, but there is in this case the fear that the potentiometers are wrongly sensed. Alternatively, a strain gauge for detecting strain of a mechanical weak point of the handle assembly is sometimes used, but there is also the fear in this case that these sensors are wrongly sensed.